Have a child who is obsessive about making sure we follow the plan

My 6 year old refuses to deviate from a plan or schedule once it’s been agreed. For example if I tell her this Saturday we will go to eat donuts and then go to the zoo. And if we instead go to eat ice cream and then go to the zoo. Or if we go to the zoo first and then eat donut (flipped order of event), she will go into a ballistic melt down. She gets upset and explains that it’s just not what we planned. I think this type of obsessive compulsive need to follow a schedule behaviour is worrying and unhealthy. Besides that she doesn’t show any other behavioural issues. What could be up?